With combined Positron Emission tomography (PET)-Magnetic Resonance (MR) devices the same volume is to be mapped simultaneously in two ways: Physiological processes are to be represented and at the same time the localization of the physiological processes in the body with high accuracy is to be possible. To do this, corresponding anatomical information must be available. For the physiological information Positron Emission Tomography (PET) images are made, for the anatomical information Magnetic Resonance (MR) images are made.
For a PET image the 511 keV photons emitted from the area under examination are verified with scintillation counters. These essentially comprise a scintillation crystal such as Bismuth Germanate (BGO) for example, onto which a high-sensitivity photodiode, e.g. an Avalanche Photo Diode (APD), is glued. Their output signals are processed in a directly-connected preamplifier and routed outwards. The noise is removed from the processed signal and for identification of true 511 keV photons in the spectrum the energy of the signal must be determined.
For an MR image strong magnetic fields are generated in the area under examination, in which the spins of the atomic nuclei align themselves. After irradiation of a high-frequency field into the area under examination through which the spins are disturbed in their previously ordered alignment, the decay behavior of the high frequency radiation emitted when the spins return to their aligned state is investigated. For local resolution parallel to the magnetic field lines gradient fields are superimposed onto the main magnetic field.
Since PET and MR imaging are undertaken at the same time at the same location, the PET components must be insensitive to the strong magnetic fields that are generated by the MR components. In particular in such cases the gradient fields of the MR imaging unit are to be considered which induce currents into the PET detector electronics. If for example preamplifiers are used for the APDs, the connections between APD and preamplifier can capture signals generated by the gradient fields. Such faults can never be entirely excluded, even by particular arrangements of the components in the electronic circuits.
The noise signals form the background in the PET spectrum, onto which the individual PET signals are superimposed. An especially large proportion of the background is contributed in such cases by the switching noise of the amplifiers for the gradient magnets. The faults caused by the switching noise are of low frequency compared to the actual PET signals and make themselves evident as drift or as low-frequency fluctuations of the zero point or the base line.
In the prior art, for improved detection of the pulse heights of the PET signal, an attempt is made to keep the base line constant. This is designed to eliminate an “offset” of the signal, through which otherwise the energy resolution of the PET components would be adversely affected. It is not always possible to keep the base line constant however since not all influences are predictable. In addition the stabilization of the base line requires a high outlay.